Feedback control of morphology evolution in metal electrodeposition by optical means

Control of morphology and in particular of roughness, is critical in metal electrodeposition. Morphology control is traditionally achieved by indirect chemical methods implying addition agents whose mechanisms of action are often rather poorly known, thus rendering the stability of processes questionable. In this paper we mean to tackle some basic problems paving the way to direct closed-loop control of morphology. The single components of a possible control loop are developed conceptually and implemented in actual devices and routines. Proper closed-loop operation of the system is on its way and will be reported in a subsequent paper. An optical method meant to directly quantify surface-roughness is devised. IR radiation scattering by reflecting surfaces is sensitive to roughness in the range typically relevant to electroplating. Reflected IR intensity at suitable frequencies is used to transduce in-situ time-dependent electrode roughness. Quantitative correlations are proposed and checked against samples of calibrated roughness both ex-situ and in-situ. A simple analytical model of roughness evolution during galvanostatic electroplating is proposed for control purposes. A suitable algorithm for automatic galvanostatic control is developed.